Plant material.

A total of 35 annual (cool- and warm-season) annuals species were selected for validation of N recommendations over two growing seasons (Table 1). In addition, ‘Montego Yellow’ or ‘Montego Purple’ snapdragon (Antirrhimum majus) and ‘Profusion Cherry’ zinnia (Zinnia elegans ×angustifolia), which are cool- and warm-season annual species evaluated previously by Shurberg et al. (2012b), were planted as reference species in both years to ensure a consistent quality response to the N fertilizer rates across studies. Annual plant species were received as plugs from Knox Nursery (Winter Garden, FL), as seed from Park Seed (Greenwood, SC) or Harris Seed (Rochester, NY), or in 4-inch pots from American Farms (Naples, FL). Cool- and warm-season plugs and seeds were grown in 4-inch azalea pots (Reb Plastic Inc., Orlando, FL) for ≈6 (plugs) or 8 weeks (seed) before transplanting into raised beds, to ensure that all plants were of comparable size at transplanting. To maximize consistency in N contributions during rearing, annual species were fertilized with a liquid 21N–3.15P–5.8K acid special fertilizer (Peter’s Professional^®; Everris NA, Dublin, OH) at an N rate of 100 ppm twice per week for 1 week, then at an N rate of 200 ppm twice weekly until transplanting.

Table 1.

Common name, scientific name, cultivar, and source of cool and warm-season annual plant materials used to evaluate aesthetic quality response to nitrogen (N) fertilizer rate for landscape-grown plants in west-central Florida (U.S. Department of Agriculture hardiness zone 9b).

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Three herbaceous perennials [blanket flower (Gaillardia pulchella), goldenrod (Solidago chapmanii), and mondo grass (Ophiopogon japonicas)] were also selected for validation of N rate recommendations. ‘Mystic Spires’ salvia (Salvia longispicata ×farinacea) was evaluated previously by Shurberg et al. (2012a) and was planted as a reference species to ensure a consistent quality response to the N fertilizer rates across studies. All perennial species were received in 1-gallon containers; blanket flower and goldenrod were received from Sweetbay Nursery (Parrish, FL); mondo grass and salvia were received from Landscape Flower Growers, Inc. (Riverview, FL).

Experimental design.

Raised beds (10 × 50 × 0.5 ft) containing subsoil fill material (St. John’s fine sand, Sandy, siliceous, hyperthermic Typic Alaquods) [U.S. Department of Agriculture (USDA), 2004] were constructed at the UF-IFAS Gulf Coast Research and Education Center in Wimauma, FL (USDA hardiness zone 9b). An initial sample of soil fill was analyzed for pH and soil nutrient content before transplanting as reported previously in our studies comparing annual and perennial growth to N fertilization under similar growing conditions in central Florida (Shurberg et al., 2012a, b). Sixteen annual plants of each species were planted in a square configuration (four plant species × four replicate plants spaced 1 ft apart) in a total of nine raised beds (plots). Cool-season annuals species were planted into raised beds on 29 Oct. 2010 (year 1) and 6 Dec. 2011 (year 2). Warm-season annual species were planted into raised beds on 25 May 2011 (year 1) and 3 May 2012 (year 2). Fifteen herbaceous perennial plants of each species were planted in a rectangle configuration (five plant species × three plant replicates spaced 1 ft apart) in the remaining area within the same nine plots on 4 Mar. 2011.

Irrigation was applied through nine drip lines (Jain Irrigation, Winter Haven, FL) installed on 1-ft spacing with 8-inch spacing between emitters and a flow rate of 0.65 gal/100 ft² per minute. Plants were irrigated for 25 min, three times per week starting at 0830 hr. Cumulative rainfall was 82.6 inches from May 2010 to Apr. 2012 with average temperatures ranging from 51 to 81 °F, maximum temperatures of 96 °F, and minimum temperatures of 22 °F (University of Florida, 2014). Weeds were manually removed or treated with glyphosate (Round-Up^®; Monsanto, Creve Coeur, MO) as needed. No mulch was added to beds to minimize outside N contributions. All plant species were fertilized at three different N rates, not to exceed current recommendations for landscape fertilization (Florida Department of Environmental Protection, 2002). Plants were fertilized every 12 weeks (based on the release rate of the fertilizer published by the manufacturer) with a 42N–0P–0K polymer-coated, controlled-release N source (Polyon; Harrell’s, Lakeland, FL) at the following annual N rates: 3, 5, or 7 lb/1000 ft² (annuals) and 1, 3, or 5 lb/1000 ft² (perennials).

Plant aesthetic quality ratings.

Aesthetic quality ratings (0–5), which considered canopy density, canopy dieback, flowers, and general form, were assigned every 6 weeks to the middle plants (four annuals and three perennials) from each planting to avoid potential edge effects. Quality ratings were applied on the following scale: 0 = dead, 1 = poor (low canopy density, few to no flowers, and/or substantial chlorosis), 2 = below average (significant dieback or chlorosis, lower density, and flowering), 3 = average or acceptable (moderate dieback, limited chlorosis, adequate form and flowering), 4 = above average (minimal dieback, healthy plant, above average density and form), and 5 = outstanding (dense leaf canopy, high-quality flowers, no nutrient deficiencies or dieback) (Shober et al., 2009). Annual species were evaluated for 18 weeks and perennial species were evaluated for 54 weeks.

Statistical analysis.

The experiments were designed as a completely randomized design, with N rates applied randomly over nine raised bed plots containing either four (annuals) or three (perennials) plants of each species per bed. Each plant type (cool-season annuals, warm-season annuals, herbaceous perennials) and species was evaluated separately. Plant aesthetic quality data for all plant species was analyzed using PROC GLIMMIX (SAS version 9.2; SAS Institute, Cary, NC) by species and weeks after planting (WAP), with N rate as a fixed effect. Plant cluster × treatment was included as a random effect using a normal distribution and an identity link function. No planting year × N rate interaction was noted for annuals reference-species quality; therefore quality data from both years were combined for analysis. Pairwise comparisons for aesthetic quality were conducted with the Tukey’s honestly significant difference test at a significance level of α = 0.05. Data were checked for normality by examining histogram and normality plots of the conditional residuals.

Cool- and warm-season annuals.

Previous research at the central Florida experiment location evaluated the growth responses of cool and warm-season annuals to N fertilizer rate (Shurberg et al., 2012b). However, the goal of this research was to validate the suggested N fertilization rates to maintain acceptable quality cool and warm-season annuals. Aesthetic quality snapdragon, foxglove (Digitalis purpurea), and kale (Brassica oleracea) were affected by fertilizer rate (Table 2). Higher quality ratings were assigned to snapdragon receiving 7 lb/1000 ft² per year N compared with plants receiving 3 or 5 lb/1000 ft² N per year at 6 WAP and 3 lb/1000 ft² N per year 12 WAP. However, snapdragon consistently received quality ratings of 3 or greater (all weeks in both years) regardless of N fertilizer rate (Table 2). At 6 WAP, foxglove had higher quality when fertilized with N at the 7 lb/1000 ft² per year rate compared with plants that received N at an annual rate of 3 lb/1000 ft². However, no further fertilizer rate effects were noted for foxglove quality; all fertilizer rates produced foxglove plants with a quality rating of 3 or greater (Table 2). Aesthetic quality of kale was greater when plants were fertilized with N at an annual rate of 5 and 7 lb/1000 ft² at 12 WAP and 7 lb/1000 ft² at 18 WAP than when plants received only 3 lb/1000 ft² per year N (Table 2). There were no effects of N rate on aesthetic quality for any other cool-season annual species. Aesthetic quality ratings for cool-season annual receiving 3 lb/1000 ft² N per year ranged from 0.0 to 4.8 (Table 2). The quality of several cool-season annual species at 18 WAP [e.g., gypsophila (Gypsophila muralis), impatiens (Impatiens wallerana), ornamental kale, linaria (Linaria sp.), nasturtium (Tropaedum nanum), and statice (Limonium sinuatum)] was below average, regardless of N fertilizer rates; all gomphrena (Gomphrena globosa) were dead by 12 WAP (Table 2). We suspect that environmental conditions, cultivar selection, or plant age may have impacted growth. Nitrogen fertilizer rate did not appear to be the major contributing factor to the low quality or death of these cool-season annual species.

Table 2.

Plant quality response of selected landscape-grown cool season annual species to nitrogen (N) rate in 2 years in west-central Florida (U.S. Department of Agriculture hardiness zone 9b).

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Nitrogen fertilizer rates did not affect the aesthetic quality of warm-season annual species, with the exception of ornamental basil (Ocimum basilicum) (Table 3). Ornamental basil produced higher quality plants when supplied with N at the annual rate of 5 lb/1000 ft² compared with plants receiving the 3 lb/1000 ft² annual N rate at 6 WAP or the 7 lb/1000 ft² annual N rate at 12 WAP. However, aesthetic quality of ornamental basil was unaffected by N rate at 18 WAP. Warm-season annual quality ratings ranged from 0.0 to 4.7 when plants were fertilized at the 3 lb/1000 ft² annual N rate. The quality of several species [e.g., celosia, (Celosia spicata), cosmos (Cosmos sulphureus), lavender (Lavandula angustifolia), nicotiana (Nicotiana alata), niermbergia (Nierembergia hippomanica), and salvia (Saliva splendens)] was below average (or plants were dead) toward the end of the evaluation period, regardless of N fertilizer rates (Table 3). We suspect that other factors, such as species selection, growing environment, or plant age, contributed to the low quality or death of these warm-season annual species.

Table 3.

Plant quality response of selected landscape-grown warm-season annual species to nitrogen (N) rate in 2 years in west-central Florida (U.S. Department of Agriculture hardiness zone 9b).

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Wright et al. (2009) reported that growth and dry weight of four annual landscape plants [‘Cocktail Vodka’ begonia (Begonia ×semperflorens-cultorum), ‘Red Hot Sally’ salvia, ‘Bonanza Yellow’ marigold (Tagetes erecta), and ‘Cooler Pink’ vinca (Catharanthus roseus] increased as N fertilizer rate increased from 0 to 2 lb/1000 ft² N per year; but, the authors reported no effect of N rate on aesthetic quality. However, their study was conducted in Virginia and may not be directly comparable to central Florida. Shurberg et al. (2012b) reported that the application of >8 lb/1000 ft² of N annually in central Florida was required to maximize size index and dry biomass of six species of annual landscape plants [‘Profusion Cherry’ zinnia, ‘Cora White’ vinca, ‘Golden Globe’ melampodium (Melampodium divaricatum), ‘Telstar Crimson’ dianthus (Dianthus chinensis), ‘Delta Pure Violet’ pansy (Viola wittrockiana), and ‘Montego Yellow’ snapdragon]; however, acceptable aesthetic quality was achieved with much lower N rates of (2 to 6 lb/1000 ft² N per year), depending on species.

Perennials.

As reported for the annual species, the aesthetic quality ratings of few perennial species were affected by N rate (Table 4). Again the goal was to validate N fertilizer rates determined in previous work by Shurberg et al. (2012a) on perennial plant growth to N fertilization rate. Differences in the aesthetic quality of perennial species due to N rate treatments were limited to blanket flower (18 WAP) and salvia [54 WAP (Table 4)]. Higher quality blanket flower and salvia plants were produced when plants received 3 or 5 lb/1000 ft² N annually compared with plants fertilized with N at the 1 lb/1000 ft² per year rate (Table 4).

Table 4.

Plant quality response of selected landscape-grown herbaceous perennial species planted 4 Mar. 2011 to nitrogen (N) rate in 2 years in west-central Florida (U.S. Department of Agriculture hardiness zone 9b).

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Chen et al. (2011) reported that application of N fertilizer improved both growth and visual quality of six perennial landscape species in Louisiana up to a rate of 2 lb/1000 ft² N per year. Nitrogen fertilizer supplied at the higher rate of 4 lb/1000 ft² N per year continued to push growth of cigar plant (Cuphea ignea), ‘New Gold’ lantana (Lantana ×hybrid), and ‘Goldsturm’ rudbeckia (Rudbeckia fulgida), but did not affect visual quality. However, results might vary in central Florida. Shurberg et al. (2012a) found that higher rates of N fertilization in Florida (>8 lb/1000 ft² N per year) could increase growth of five perennial landscape species [bush daisy (Gamolepis chrysanthemoides), ‘New Gold’ lantana and ‘Mystic Spires’ salvia, ‘Evergreen Giant’ liriope (Liriope muscari), and ‘White Christmas’ caladium (Caladium bicolor)], but that acceptable aesthetic quality was achieved at low rates of 0 to 4 lb/1000 ft² N per year, depending on species.